Ultra-fast extraction of metals from a printed circuit board using high power ultrasound in a calcium chloride-based deep eutectic solvent

Abstract

The increase of digitization, alongside the growth in consumer electronics and shortened life cycles, has led to a significant global increase in the volume of electronic waste (e-waste). Printed circuit boards (PCBs) are the most valuable components within e-waste due to the higher content of valuable (critical) metals compared to ores. Although some of these metals can be recycled by traditional methodologies, e.g., pyro- and hydrometallurgy, the utilisation of deep eutectic solvents (DESs) offers the opportunity to recover metals from e-waste with higher selectivity and lower toxicity. Yet, DESs are often characterised by very high viscosity, which slows dissolution kinetics. This study investigates the effect of ultrasound on the catalytic dissolution of metals, in the deep eutectic solvent formed from calcium chloride hexahydrated and ethylene glycol. Copper(II) chloride was considered as an oxidising agent, which has high solubility and reversibility in this solvent. High ultrasonic power prevented passivation leading to an enhancement of copper dissolution at a rate 10 000-times faster than without ultrasound. Thus, leaching rates determined with an optical profiler showed that metals can be extracted from a PCB much faster (7 μm min⁻¹) with high power ultrasound rather than in silent conditions (0.6 μm min⁻¹), and this is discussed in terms of chemical oxidation, passivation and cavitation activity.